A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. Each natural frequency is associated with a particular mode, each of which is a different combination of vibrational deflections such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, and so forth. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. However, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. This may occur even if the natural resonance frequencies of the blade are not near the harmonics of the running speed of the turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in extensive damage, thus shutting the machine down and requiring a costly forced outage for extensive repair. In order to avoid the aforementioned problem, detailed testing is performed prior to operation of a machine to ensure that blades will not resonate during normal operation.
It is also desirable to monitor rotating blades during operation in order to identify vibration problems which develop after a turbo machine is put in use. This on-line evaluation is necessary in part because evaluations performed prior to actual use do not subject the blades to the same temperature, pressure, fluid flow and rotational conditions associated with adjacent vanes and blades, and other conditions which are experienced during normal operations. Continuous monitoring of blade vibrations is also important in order to detect new vibrations which signal structural changes. If any of these vibrations escape detection, developing fractures will likely lead to extensive damage and costly down time while the machine undergoes repair. For example, it is known to use non-contacting proximity sensors or probes to detect blade vibrations. The probes detect the actual time-of-arrival of each blade as it passes each probe. The difference between the actual time-of-arrival of each blade and its expected time-of-arrival, determined with the use of an additional probe which tracks rotation of the turbine wheel, produces a signal containing blade vibration information. Fourier analysis is applied to this signal to extract the blade vibration frequencies and amplitudes.
In order to limit vibrational stresses in the blades, various structures may be provided to the blades to form a cooperating structure between blades that serves to dampen the vibrations, and to otherwise make the blade structure non-responsive to flow excitation generated during rotation of the rotor that might excite the blade. For example, in a known steam turbine blade construction, each turbine blade may be provided with an outer shroud portion located at an outer edge of the blade and having front and rear shroud contact surfaces. The front and rear shroud contact surfaces of adjacent blades are normally separated by a small gap when the rotor is stationary, and move into contact with each other as the rotor begins to rotate to form a substantially continuous circumferential shroud structure. The circumferential shroud structure substantially raises the natural frequencies of all modes of vibration and thus reduces the number of vibrational modes that can interact with the lower harmonics of the rotor rotational speed as well as for those due to flow induced nonsynchronous blade excitations. Moreover, the circumferential shroud structure tends to respond substantially at a single vibrational frequency for each mode of vibration, i.e., the frequency that is associated with a nodal diameter pattern in the blade row, where the number of nodal diameters is equal to the number of the harmonic of the running speed at which the vibration occurs.
With regard to nonsynchronous vibration that is typically induced by aero elastic (flow) effects, i.e., not a multiple of shaft frequency, shrouding the blades and placing points of contact, such as snubbers, at several points of contact along the length of each blade severely restricts the conditions under which the blades will accept energy from the flow excitation forces, for generating blade vibration. That is, not only must the aero elastic excitation forces have the correct frequency, they must also have the correct restricted set of nodal patterns. For a free-standing blade, only the frequency of the excitation force need match. The shrouded blade row is thus generally unresponsive at most flow excitations, even when the frequency content in the flow energy matches the resonance frequency of the blade. Further, as a result of the increased stiffness in the system caused by the coupled shrouded blades, each nodal pattern shifts the fundamental free standing blade frequencies from what they normally would be without the increase in stiffness.
In known systems for monitoring and analyzing vibrations in shrouded blade structures, where the shroud is made integral with the blade, the shroud may be provided with targets that are placed in the shroud, where one target is generally provided to each shroud portion associated with a blade, such that each target corresponds to a blade. In most field test and on-line applications, a single sensor may be provided for sensing the arrival of each target as the target passes the sensor. The data is analyzed on the basis of data identified with each target, i.e., each blade, such that the vibration characteristics of each target location are individually analyzed, based on multiple rotations of the blade row, to characterize the vibration characteristics of the coupled shroud structure.